This invention relates generally to motor systems and, particularly, to an improved gate drive for a single phase electronically commutated motor.
In general, a motor such as an electronically commutated or brushless motor has permanent magnets mounted on its rotor. The stator of such a motor has a plurality of teeth and wire-wound coils on the teeth which, when energized with current, interact with the permanent magnet rotor to produce positive or negative torque, depending on the direction of the current with respect to the polarity of the magnets. The polarity of the magnets relative to the stator winding alternates when the rotor moves unidirectionally. Thus, alternating the direction of the stator current in synchronism maintains a constant direction of torque. An electronic inverter bridge typically controls energization of the stator winding for controlling the direction and amount of torque produced by the motor as well as the rotor shaft speed.
For example, a brushless DC motor system typically has an inverter bridge for driving the motor. The inverter bridge has a number of power switching devices with a flyback diode coupled to each of the devices and is used to connect the motor's windings to a power supply through a power supply link. The power supply link has positive and negative rails connecting the motor to the power supply. Generally, such a motor system provides pulse width modulation of the power to the windings by turning on and off one of the power switching devices. Presently available gate drives for full bridge circuits include RC circuits in which the change in voltage across a capacitance controls the conduction of the bridge circuit's diagonally-positioned transistors. These drives attempt to prevent shoot-through currents by delaying the drive signals that alternately switch the transistors, i.e., providing a "dead time" between commutation switching events. The conduction state of each of the upper two transistors of the bridge circuit often depends on the conduction state of their respective diagonally-positioned lower transistor (i.e., the lower transistor on the opposite side of the load). A capacitance connecting the bases of the two inputs of the lower transistors delay the conduction in one diagonal until after the other diagonal is turned off.
In general, brushless dc motors are disclosed in, for example, U.S. Pat. Nos. 5,423,192, 4,933,584 and 4,757,241, all of which are commonly assigned with the present invention described herein and the entire disclosures of which are incorporated herein by reference. In particular, single phase motors are disclosed in, for example, U.S. Pat. Nos. 5,483,139, 5,465,019, 5,140,243, 4,724,678, 4,635,349, 4,626,755, 4,313,076 and 3,134,385, all of which are commonly assigned with the present invention described herein and the entire disclosures of which are incorporated herein by reference.